The occurrence of thrombosis at polymer-blood interfaces presents major difficulties in the development of blood compatible materials. It has been shown that thrombus formation on a biomaterial surface is largely dependent on the ability of a surface to elicit platelet spreading. It is hypothesized that the adhesivity between spread platelets and a surface determines whether the spread platelets are able to support thrombus growth in the shear field of the flowing blood or they are eliminated from the surface. Thus, the ability of a biomaterial to induce platelet spreading and the platelet-surface adhesivity can serve as useful parameters for predicting surface thrombogenicity. These parameters, however, have not been fully examined yet. The main objective of this study is to achieve the complete characterization of platelet behavior at biomaterial-blood interfaces. Platelet behavior, such as the kinetics and extent of platelet spreading, on various biomaterials will be examined using a video-enhanced differential interference contrast light microscope. The adhesive strength between platelets and a surface will be determined by the microjet shearing method. The interaction of platelets with a protein layer which is formed on biomaterials upon blood exposure will be studied using antibody exclusion methods, immunogold staining techniques, and interchromophore energy transfer measurements. In addition, a new in vitro technique which simulates in vivo thrombus formation will be developed. The formation of thrombus-like mural platelet aggregates in vitro can serve as a model for the in vivo thrombus formation. This new method will be able to screen large number of polymers for their potential thrombogenicity in the same condition and generate the same information as that from animal experiments, at least for the acute surface-induced thrombosis. Completion of this study will produce valuable information for the design of truly blood compatible materials.